The present invention relates to nuclear power plants, and in particular, to on-line monitoring of control rod positions relative to regulatory requirements for short term, long-term, and transient insertion limits.
Commercial nuclear power plants are subject to comprehensive regulatory compliance covering virtually every phase of nuclear reactor operation. Many of these regulatory constraints are manifested in the form of so-called "Technical Specifications", which are an integral part of the operating license for the power plant. Each vendor of a nuclear steam supply system (NSSS), achieves compliance with the technical specifications, by formulating and justifying operating procedures for approval by the regulatory authorities.
In pressurized water nuclear power plants (PWR plants), one type of Technical Specification concerns the accumulated time during which control rods are present in the reactor core. As is well known, control rods serve two important functions. The extent of insertion directly affects the gross power level in the reactor core. Another function is to control the local distribution of power in the core, thereby avoiding high localized power peaking, relative to the average power generated in the core. The prolonged insertion of particular control rods in the core, especially during periods of relatively high power, can have two detrimental effects long term. First, the pattern of fuel consumption can be distorted to the extent that upon removal of these rods, new, previously unpredicted local power peaking or power oscillations may arise. Furthermore, control rods can prematurely lose effectiveness over time.
It is also well known that individual control rods can be ganged together as an assembly for insertion and removal by a single drive mechanism, and that a plurality of assemblies, such as four or eight, can be controlled as a group for substantially simultaneous movement into and out of the core. Four or five groups are typically programmed for staggered insertion and withdrawal from the core (unless, of course, all groups are to be dropped simultaneously to trip, or "scram", the reactor). For purposes of the present disclosure, a cluster of control rods which are moved by a single drive mechanism, are referred to as a "control element assembly" (CEA), whereas a plurality of CEA's which are controlled for substantially simultaneous movement into and out of the reactor, are referred to as a "CEA group".
According to one approach for compliance with Technical Specifications, plant Limiting Conditions of Operation (LCO) are established to impose operational constraints with regard to CEA rod group insertions and thereby assure that the design bases which underlie the Technical Specifications are not violated. These limitations are typically characterized in terms of restrictions imposed on CEA rod group insertions between the Long Term Steady State Insertion Limit (LTSSIL) and the Transient Insertion Limit (TIL). These restrictions are typically expressed in terms of either clock hours, or effective full power days (EFPD) of exposure. An EFPD is the exposure equivalent of 24 hours at the licensed full power operation of the reactor. In addition, restrictions are imposed upon exceeding the Short Term Steady State Insertion Limit (STSSIL) under certain conditions. In a PWR, all CEA's are typically out of (above) the core at full steady state power, and are inserted downwardly into the core to reduce power level. Typical examples of limiting conditions of operation are set forth in the following Table 1.
TABLE 1 ______________________________________ ROD GROUP OPERATIONAL LIMITATION APPLICABILITY (LCO) CRITERIA ______________________________________ Regulating Insertion between STSSIL and Limit to 4 hours TIL per 24 hour interval Regulating Insertion between LTSSIL and Limit to 5 EFPD TIL per 30 EFPD interval Regulating Insertion between LTSSIL and Limit to 14 EFPD TIL per 365 EFPD interval Regulation Insertion beyond the STSSIL Take Prescribed with COLSS out ot service Action within 1 hour Part Strength Insertion between LTSSIL and Limit to 7 EFPD TIL per 30 EFPD interval Part Strength Insertion between LTSSIL and Limit to 14 EFPD TIL per 365 EFPD interval ______________________________________
These restrictions limit the duration (in terms of hours) that CEA rods can be positioned between the STSSIL and the TIL, and the amount of CEA exposure which can be accumulated (in terms of Effective Full Power Hours) for insertions between the LTSSIL and the TIL. The graph of FIG. 20 depicts typical operational regions bounded by these insertion limits.
The LTSSIL is a position limit in which there is no restriction for CEA rod insertions which are above this position. However, CEA rod insertions below this position and bounded by the TIL are constrained to the limits of CEA exposure as noted in Table 1.
The STSSIL is a position limit below (i.e., greater than) the LTSSIL in which further restrictions on insertion (time duration--as opposed to CEA exposure accumulations) are imposed on CEA rod insertions which are below this position and bounded by the TIL. These limits are noted in Table 1.
The TIL is a position limit below the STSSIL which CEA rod insertions must not exceed. This limit is designed to allow for plant maneuvering using CEA insertions (as long as the CEA's do not go below this limit and as long as they maintain the CEA exposure and time limit durations for insertion as previously noted). Should CEA's be inserted below the TIL, the plant annunciator system normally outputs an alarm message and the operator must then take corrective action (such as--restore the CEA rods to within the prescribed limits within a defined time period; or reduce plant thermal power).
It is conventional to identify groups of CEA's beginning with number 1 and proceeding, e.g., to number 5 according to the order in which they are withdrawn from the core in a zero power condition at which all CEA groups are fully inserted. The corollary is that in the initial condition where the reactor core is at full power, with all rods out (the most desirable operating condition), Group 5 is the first to be inserted, followed by 4, 3, etc.
The Long-Term Steady State Insertion Limit is shown in FIG. 20 as a vertical line extending through range of 1.0-0.2 power fraction and (when projected) intersecting the Group 5 insertion representation bar at an insertion distance of 108 inch (274 cm), out of a total group rod length of 150 inches (381 cm). Because Group 4 and subsequent groups follow in staggered relationship, it is clear that whenever Group 5 is positioned in the core anywhere within the Steady State Insertion Limit, no other Groups are in the core. It is evident that Group 4 does not begin entering the core, until Group 5 is at the 60 inch (152 cm) withdrawal position (i.e., 90 inches (229 cm) of insertion).
The Short Term Steady State Insertion Limit for Group 5 is also shown in FIG. 20 as a vertical line which has an upper limit at a power fraction of 0.75 and extends downward to 0.25, and intersects the Group 5 bar at the 60 inch (152 cm) withdrawal position. Thus, it can be appreciated from FIG. 20, that the Group 5 Short Term Steady State Insertion Limit, would not be accompanied by a Short Term Steady State Insertion Limit for any other Group.
On the other hand, the Transient Insertion Limit allows for a variety of CEA insertion configurations including the fifth and fourth Groups fully inserted and the third Group inserted at the 60 inch (152 cm) withdrawal position. Not all configurations are permitted at every power level, however, i.e., the greater extent of Group insertion, the lower the permitted power level even during a transient.
Thus, it may be appreciated that the LCO's impose concurrent limitations on insertion. For example, even if the CEA groups have not reached the limit of 5 EFPD per 30 EFPD interval, for insertion between the LTSSIL and the TIL, desirable repositioning of the Groups may be foreclosed by the further requirement that insertion between the STSSIL and the TIL must not exceed 4 hours per 24 hour interval.
The foregoing operational requirements are presently maintained by manual surveillance. The inventor has concluded that this approach has the following shortfalls which are remedied by the present invention:
1. Manual monitoring is cumbersome and prone to human error. PA1 2 There is no automatic method to display and analyze the monitored data which, in turn, reduces the situational awareness for the operator of the existing accumulated CEA group exposures relative to the operational limits. PA1 3. There is no automatic early notification of approach to operational limits so that corrective action can be taken prior to exceeding an operational limit. PA1 4. There is no automatic alarm notification when the operational limits are exceeded so that corrective action may be immediately initiated. PA1 5. The resolution of the manually recorded data is coarse. PA1 6. Manual recording of accumulated EFPD and hours for CEA rod group exposures does not conveniently lend itself to monitoring a contiguous data interval or window. This may result in the selection of discrete monitoring intervals which are sequential. Such discrete monitoring intervals can lead to potential circumscribing of the intent of the operational limits. For example, the restriction of 5 EFPD per 30 EFPD will seemingly be satisfied by two sequential monitoring intervals in which 4.5 EFPD exposure occurs during the last 4.5 days of the first monitoring interval (of 30 EFPD) and in which 4.5 EFPD exposure occurs during the first 4.5 days of the following monitoring interval (of 30 EFPD). Each monitoring interval seemingly satisfies the restriction of 5 EFPD per 30 EFPD interval but, in fact, 9 contiguous EFPD of exposure have occurred. If the starting period of the first monitoring interval was advanced 5 EFPD, then the total EFPD exposure for the first monitoring interval would have been 9 EFPD (rather than 4.5 EFPD) which exceeds the operational limit. In this example, the operational limits were either complied with or violated depending upon the happenstance of when the start of a discrete monitoring interval was chosen.